Optical coherence tomography (OCT), though used in many medical and biological fields for medical imaging, is particularly suited to ophthalmic imaging because it is non-invasive and can be performed even through the anterior structures of the eye to obtain a 3-D image of the fundus oculi (i.e. the retina of the eye).
Various ophthalmic apparatuses that use optical apparatuses are known in the art, such as an anterior ocular segment imaging apparatus that images the anterior portion of the eye; a fundus camera that takes an image of the fundus of the eye; a scanning laser ophthalmoscope (SLO) for taking an image of the fundus, and so on. Ophthalmic use of OCT is also well-known and described in the art, such as in US2012/0033181 (KK Topcon) and in D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W. G. Stinson, W. Chang, M. R. Hee, T. Flotte, K. Gregory, C. A. Puliafito and J. G. Fujimoto, “Optical Coherence Tomography”, Science 254, 1178-1181 (1991).
Partial coherence interferometry for use in ophthalmology is discussed in A. F. Fercher, C. K. Hitzenberger, G. Kamp, S. Y. El-Zaiat “Measurements of intraocular distances by backscattering spectral interferometry,” Opt. Comm. 117, 43-48 (1995).
Ophthalmic use of spectral OCT is described in M. Wojtkowski, A. Kowalczyk, R. Leitgeb, and A. F. Fercher, “Full range complex spectral optical coherence tomography”, Opt. Lett. 27, 1415-1418 (2002).
OCT is particularly useful because it can obtain tomographic images in the depth direction of the fundus (for example, to obtain a 3-dimensional image of the fundus structure) whereas other imaging apparatuses image only the surface of the fundus. OCT uses low-coherent light with multiple wavelengths as a measurement beam to irradiate the fundus of the eye to be examined. The various wavelengths penetrate into the fundus by a different amount before reflecting back into the OCT's optical system. This reflected beam interferes with a reference beam that corresponds to the measurement beam and the resultant interference beam is detected to build up a tomographic image in a depth-direction. The greater the wavelength bandwidth, the greater the resolution of the resultant tomographic image because different wavelengths penetrate the fundus to different depths and reflect differently from the fundus.
Each depth-direction scan is referred to as an “a-scan”. A plurality of a-scans in a single scan line (defined by the scanning direction of the OCT apparatus) is referred to as a “b-scan”. A plurality of b-scans may be combined to build up a 3-D image. A “tomographic image” is a cross-sectional image of the fundus oculi and may follow a b-scan line or it may be perpendicular to the b-scan direction or it may be at any angle in between when it is generated by the interpolation of the 3-D volume of the fundus oculi using the OCT b-scans. In the present discussion, using Cartesian coordinates, the b-scans extend in the x-direction, the thickness of the fundus is in the z-direction and the y-direction is in the fundus plane but perpendicular to the b-scanning direction.
In order to provide enough information to enable a confident diagnosis of eye health, OCT manufacturers have combined OCT (for depth-direction tomographic imaging) with fundus imaging (for fundus observation) and sometimes anterior eye imaging (for anterior eye observation). These may even be combined in the same device.
Advantage of including anterior eye images is that such an image may be used to align the optical paths of other imaging devices with the iris of the eye. Further to this, anterior eye disease such as cataracts or other opaque portions on the lens or iris that may cause shadows on the fundus can be seen and the OCT measurement beam can be directed to a position on the lens or iris of the eye other than those positions with opaque portions.
An advantage of including a fundus image is that it can help with alignment of tomographic images. A mark superimposed on a fundus image may indicate the position of a tomographic image, the tomographic image being perpendicular to the plane of the fundus image. A fundus image may even be marked by an ophthalmologist to indicate where on the fundus they would like a tomographic image to be scanned or to indicate which fundus cross-section they would like to view.
Taking a fundus image has a further advantage of helping to align tomographic images both with the fundus so that an ophthalmologist knows where on the fundus a particular tomographic image has been taken and also with each other so that a 3-D image may be correctly assembled from a plurality of tomographic images. Furthermore, a fundus image may be used to ensure that a-scans within a b-scan are aligned to help to ensure a consistent b-scan.
The reason that alignment is a concern is that during scanning by an OCT apparatus, a person's eye is prone to movement. This movement may be blinking or saccades or some other form of voluntary or involuntary movement.
There are different ways in which eye movement is corrected or compensated for. One type of correction involves tracking the movement of the eye using a fundus image as a tracking image, for instance, and adjusting the position of the OCT measurement beam in real time to maintain the same location of an OCT scan with respect to the tracking image. This is seen in commercially-available OCT apparatuses such as Canon's OCT HS100, Nidek's RS-3000, Topcon's OCT-2000, Heidelberg's Spectralis and Carl Zeiss Meditec's Cirrus HD-OCT.
In Topcon's OCT-2000 (see also US 2012/0033181 A1), the positions of the OCT b-scans 2000 are observed relative to fundus images 1000 as can be seen in FIGS. 3A, 3B and 3C. Although the images are shown as b-scans 2000 moving with respect to the fundus images, it is in fact the fundus that is moving and the b-scans are scanning in a constant position as defined in 3-D space. The fundus image acquisition is so much faster than the OCT scans over the same area that the fundus image is much more likely to be in alignment with the fundus and its position is therefore regarded to be the same as the fundus position. The fundus moves between (or even during) b-scans to cause the relative movement artifacts. However, this relative movement is more easily depicted in the figures as b-scan misalignments with the fundus image being constant.
FIG. 3A shows an ideal scan with no misalignments as imaged by Topcon's OCT-2000. The fundus image 1000 is aligned with b-scans 2000, which are themselves equally spaced apart in the y-direction and are all straight and aligned with each other, representing alignment in the x-direction. In Topcon's OCT-2000, the b-scans 2000 contain extra a-scans 300 at each end so that when misalignment occurs in the x-direction within a certain threshold, as shown by b-scans 302 and 312 in FIG. 3B, during post-processing off-line, those b-scans can be adjusted so that only the a-scans that are in line with the fundus image 1000 are used. However, if a b-scan is misaligned by more than the threshold, such as b-scan 308 in FIG. 3B, this b-scan needs to be re-scanned.
One problem with this known apparatus is that much more information has to be processed than necessary because every single b-scan has more a-scans than will be used to form a tomographic image. This causes an extra processing burden and slows down processing.
FIG. 3C shows the known system with alignment corrected in the x-axis but not in the y-axis. Y-axis compensation is performed by comparing the obtained b-scans with the fundus image and determining where scan lines are densely packed, such as lines 304 and 306, and where scan lines are too far apart, such as between lines 318 and 320. Densely packed lines can be thinned if necessary and thinly packed lines can be interpolated to create scan lines where they are missing. If lines are separated by a distance greater than a threshold, they are re-scanned.
Another problem with some known apparatuses is that synchronisation between the fundus image and the b-scan of the OCT has to be constantly monitored, and this increases processing burden as well as increasing the time taken to create a tomographic image if no scanning is performed during the synchronisation process or if b-scans have to be corrected if an asynchronicity is found.
Specifically, there is a trade-off between a fundus image frame rate and an OCT b-scan rate. OCT b-scans are synchronised with fundus images. As a full fundus image (1000 in FIG. 3A) is scanned at a rate that is much faster than an OCT image (400 in FIG. 3A), one fundus image can be scanned in the time it takes to scan a number of a-scans within an OCT b-scan. As each fundus image is synchronised with the OCT image and this synchronisation takes time, if the fundus image capture rate increases, the number of times that synchronicity is checked per OCT image increases and the rate of capture of an OCT image necessarily decreases.
Heidelberg's Spectralis apparatus simultaneously images the fundus oculi with two beams of light. A first beam captures a fundus image to track eye movement. Using this image as a reference, the second beam is directed to the desired location on the fundus. This dual-beam technology mitigates eye motion artifacts, preventing them in the first place. During the measurement phase, this apparatus determines whether the eye is focused on a fixation target. The capture of the OCT b-scans is performed only if the eye is so focused. If not, the OCT system waits for the focus of the eye on the fixation target. Although this reduces the number of artifacts in the obtained OCT image, the measurement and image-capturing time is significantly extended.
Any kind of fundus tracking has a problem of a “feedback” delay in the feedback loop that occurs between the fundus tracking and the OCT correcting for misalignments found during the tracking. Several b-scans may be completed in the time it takes for the measurement system to determine that a misalignment between the fundus image and the OCT b-scan has occurred. Thus, several misaligned b-scan images may be taken before the misalignment is noticed. Measurement of the alignment of all of those b-scans may then be required to compensate for their misalignment.